1. Field of the Invention
The present invention relates to a method of and an apparatus for noise reduction in a multi-gradation image signal obtained by mating multi-gradationed a signal obtained by reading, through a solid-state image sensing element, an original having a half tone such as a photograph or a photofilm.
2. Description of the Prior Art
When an original having a half tone is read through a solid-state image sensing element, such as a CCD, the half tone cannot be correctly read as compared with the case of employing an image sensing tube or a photomultiplier, because of noise caused by the element itself. In order to solve such a problem, an effort has been made to reduce the noise caused by the element itself, whereas no sufficient effect has been attained since the mechanism of noise generation is not yet sufficiently understood. Since it is difficult to reduce the noise caused by the element itself, influence by the noise thus caused has been generally weakened by signal processing in, e.g., a method of reading the same portion of an original a plurality of times to average image signals of an identical pixel thereby to reduce the noise. In this case, the amount of random noise is ##EQU1## for n readings. In this method, however, the reading speed is decreased since the same portion of the original must be read a plurality of times. On the other hand, the noise can also be reduced from an image signal obtained by one reading by performing averaging between adjacent pixels, although resolution is reduced in this case.